A typical power supply in a television system such as, for example, a set-top box includes a pre-regulator that produces an output supply voltage in a filter capacitor. The output supply voltage energizes an electronic circuit of the set-top box.
For recovering from an invalid operational mode that might occur in the electronic circuit producing, for example, an objectionable picture “freeze”, the user might wish to re-initialize or reset the electronic circuit. Proper initialization or reset operation might require the output supply voltage not to exceed, for example, 0.2 volts, during at least a portion of the reset operation. This is so in order to avoid any latch-up condition in stages of the electronic circuit. Thus, the user would initiate the generation of a reset pulse. The reset pulse, in addition to performing the reset operation, also disables the pre-regulator for stopping the generation of the output supply voltage during the reset operation.
Typically, the output supply voltage is coupled to supply input terminals of post regulators such that the electronic circuit is energized directly from the post regulator. Each of the post-regulators includes, typically, an under-voltage lock-out feature. Immediately after the pre-regulator is disabled, the output supply voltage of the post regulator quickly drops to a threshold level, typically 6 or 7 volts, established by the under-voltage lock-out feature. The post-regulator will reduce the loading of the filter capacitor after reaching the threshold level. Therefore, following the post-regulator output voltage quick drop to the threshold level, the output supply voltage would not be able to continue dropping at a fast rate. Thus, if the filter capacitor were left to discharge with the reduced loading associated with the under-voltage lock-out feature, completion of the proper initialization or reset operation, and hence the subsequent normal operation, would have to be, disadvantageously, excessively extended or delayed.
Delaying normal operation after initiating the reset operation by more than 3 seconds was found irritating to the user. Furthermore, a circuit that produces the user initiated reset pulse is energized by a charge previously stored in a second capacitor. Using a large capacitance for the second capacitor to extend the time that the second capacitance voltage is maintained at a sufficiently large level might not be practical. Therefore, also for this reason, a quick completion of the initialization or reset operation might be required.